STRENGTH AND ELASTICITY OF REINFORCED CONCRETE. 165 



Mr. A. L. Johnson has proposed and largely used corrug- 

 ated bars, consisting of rolled steel bars having a ribbed 

 surface, to reinforce concrete beams which exceed a span 

 of 15 feet ; for spans between 8 and 15 feet expanded metal 

 is used to reinforce the concrete. Both the corrugated 

 bars and the expanded metal furnish a mechanical bond, 

 quite independent of the bond due to adhesion between the 

 steel and the concrete. M. Oonsidere has shown that the 

 adhesive resistance tends to yield to a soliciting force, and 

 there can be no doubt that some structures are exposed to 

 vibrations and shocks which must tend to break the ordinary 

 adhesion bond between steel and concrete. 



Fig. 16 shows the arrangement of corrugated bars in 

 Johnson's reinforcement for deep beams. 



'Fig. 17 shows the arrangement of the Kalm bars in a 

 similar beam, in which the reinforcement is inclined to the 

 vertical, with varying upward curvature approximating to 

 the lines of principal tensile stress : — 



In the Prussian regulations, just published, for reinforced 

 concrete in building construction, the following is worthy 

 of notice in regard to working stresses : — 



In the members subjected to bending, the compressive 

 stress in the concrete shall not exceed one-fifth of its 

 ultimate resistance ; the tensile and compressive stresses 

 in the steel shall not exceed 17,000 pounds per square inch. 



The following loads shall be provided for : — 



a. For structural parts subjected to moderate impact 



the sum of the live and dead loads. 



b. For parts subject to higher impact or widely vary- 



ing loads ; the dead load added to one and one half 

 times the live load. 



c. For parts subject to heavy shocks ; the dead load 



added to twice the live load. 



